Calculating kW at Panel

[SKalkbrenner]

Good afternoon everyone.

I am trying to verify my kWh usage due to several months of unexpectedly high bills!! OUCH!

I have recently (2 months) moved into a converted commercial property which started with a 100A standard service and a 100A 3 phase service. New sub-panels were added for both services, with the 3-phase sub consisting of 2 pole breakers for 240V appliances (only using 2 of 3 legs).

My question(s) involves testing the lines as they enter the building. My understanding is that if my dryer was on a 2-pole breaker, standard service panel - I would use my clamp meter on either hot wire to measure current. I would then multiply by 240 to get w and convert to kW ... If that is correct, could I not also back up and take a clamp reading of each 120V service lines and perform the same calculation (120V x I) to calculate what the standard feed is using?

Again, and I know I have a lot of "ifs" above, if the above calculations are legit, what adjustments do I need to make on my 3 3-phase feeds to make the same calculations? My dryer is currently fed from a 2-pole breaker in my 3-phase sub-panel. Am I taking a significant hit because of this vs. being in a standard panel?

My plan is to measure/calculate at each service feed and then also dive deeper into each circuit as needed.

So - let me have it. Is my plan upside down? Crazy talk? or doable?

Thanks in advance for any/all insight provided.

Steve

 

[jim dungar]

It is important to remember that you are being billed for kW per hour. Most clamp-on meters are not rated for continuous use, so you will most likely be measuring kW per minute if not per 10secs.

Too many 'what-ifs' to make hand measurements worthwhile.

 

[Jraef]

To put a finer point on that, you might come up with 10kW when you calculate based on what you read with an ammeter. But how long was your dryer running? Even if the dryer is running for an hour, the heater elements were not likely on for the entire hour, they cycle on and off repeatedly. So would your electric oven, hot water heater and any other heating appliances. You can only measure a single moment of time with an ammeter, energy is billed based on an integrated function of power across time.

If you have a previous history that shows a lower kWh use and it just suddenly jumped up higher, a common culprit is an electric hot water heater that is malfunctioning. For example it might be that it is never shutting off, the water over heats and is let out through the Pressure Relief Valve, which is often plumbed directly to a drain waste line, so you never see it.

 

[Hv&Lv]

If you really want to get into digging in and you have some time without buying a super expensive test setup or logging meter from someone like fluke, you can build your own cheap(ish), make your own...

you know anyone from the poco? Got a CT laying around at 100:5? Probably not, but anyway...you can buy one new for about $50

get an old form 3s meter or buy one for about $50. A form 3s meterbase from the supply house and your all set!
Put whatever wires you want through the CTs and measure the kWh yourself.
just remember, one leg one way, the other leg the other way through. Then cut your multiplier inhalf since your doubling your current...

 

[tom baker]

You can read the KW off the meter but there is a multiplier on the dial or display.

 

[Hv&Lv]

Yes, that’s what I’m saying, cut the 100:5 in half to make the meter multiplier 10

 

[SKalkbrenner]

Thank you everyone for your input. I totally understand I am only getting a snapshot with my clamp meter. For instance, I had my wife turn all for top elements on high plus turn the oven on, while I had the clamp on the oven circuit. I realize I registered the MAX draw - I even noticed/could tell the instant one of the small oven tops reached temp. But, at this point I am looking for a huge elephant in the room so to speak. By that, I mean if I calculate the kW and assume different times, I should be able to notice a glaring issue.

What I do not know is if my circuits on my 3 phase panel should be calculated differently.

 

[RyanEE]

I'll take a shot at answering your question.
Standard 120/240V Panel: The two incoming hot conductors are 180 degrees out of phase. Line to line voltage is 240V. Line to neutral voltage is 120V.
Standard 120/208V 3 Phase Panel: The three incoming hot conductors are 120 degrees out of phase. Line to line voltage is 208V. Line to neutral voltage is 120V. So, for the load on a two pole breaker, multiply current by 208V instead of 240V. For load on a 1-pole breaker, multiple current by 120V. For load on a 3-pole breaker, multiply current by 208*1.73. And, panel load = phase A current * 120 + phase B current * 120 + phase C current * 120.
Clear as mud?

 

[SKalkbrenner]

I'll take a shot at answering your question.
Standard 120/240V Panel: The two incoming hot conductors are 180 degrees out of phase. Line to line voltage is 240V. Line to neutral voltage is 120V.
Standard 120/208V 3 Phase Panel: The three incoming hot conductors are 120 degrees out of phase. Line to line voltage is 208V. Line to neutral voltage is 120V. So, for the load on a two pole breaker, multiply current by 208V instead of 240V. For load on a 1-pole breaker, multiple current by 120V. For load on a 3-pole breaker, multiply current by 208*1.73. And, panel load = phase A current * 120 + phase B current * 120 + phase C current * 120.
Clear as mud?

Ryan - Thank you for your reply. My 3 phase service is actually 3-Phase, 4 Wire Delta (Wild Leg). So I have 2 of the 3 legs at 120V line to N and the "wild" leg is 208V line to N. All Line-to-Line voltages are 240.

I have seen the calculation for 3-pole breakers with the 1.73 multiplier, I just wasn't sure if I had to use a similar factor using a 2-pole breaker. From your perspective, it looks like I should not. Likewise, I could calculate panel load as you noted with the only change being swap out 1 of the 120V for my 208 "wild" leg.

Just thinking out loud... I wonder how/if the calculation changes when I compare 2 different 2-pole breakers in my 3-phase subpanel. Breaker 1: 240V, fed with 2 120V lines. Breaker 2: 240V, fed with 1 120V and the "wild" leg. Thoughts?

 

[RyanEE]

Looks correct to me. I don't do much residential work, but here's how I'd approach that scenario:
Breaker 1: Line-to-line voltage at load is 240V. If no neutral, load = current at either hot * 240. If there is a neutral conductor to load, load = hot#1 current * 120 + hot#2 current * 120.
Breaker 2: Line-to-line voltage at load is 240V. If no neutral, load = current at either hot* 240. If there is a neutral conductor to load, load = hot#1 current * 120 + hot#2 (wild leg) current * 208.